This invention relates to the field anodic oxide formation and utilization and especially to anodic oxide used as an electrical insulator.
Dielectric materials are a limiting consideration in improving capacitor energy storage elements. The ability of presently available dielectric materials to provide only small increases in electrical insulation and heat conductivity are notable examples of such limitations. High energy projects in space, as encountered for example in weapons for the strategic defense initiative program, are in particular need of small light weight high energy density capacitors. Energy storage capacitors for space use and also for more conventional uses are of course desirably made to be small in volume, light in weight, of low dielectric power loss, low internal resistance or ohmic loss, small in electrical inductance, have high dielectric breakdown voltage, a low-out gassing rate and have reliability that extends through a period of ten years without replacement or attention. Capacitors of this capability are virtually nonexistent at the present time.
The solid state metal-insulator-metal capacitor, as referred to herein, has characteristics which extend toward the satisfaction of these requirements for space power and for demanding earth applications. When embodied as an aluminum-aluminum oxide-aluminum structure, capacitors of this type can provide electrical energy storage density extending into the range of 18 joules per cubic centimeter of volume, mass in the range of 3.2 grams per cubic centimeter and therefore, energy storage densities in the range of 1,000,000 joules per cubic foot of volume and 400 pounds of weight. These values are based on dielectric constants in the range of 10.0 for the thin film insulation used between capacitor electrodes.
Aluminum oxide produced by anodization, the preferred dielectric material of the present invention, has been shown to have desirable high values of thermal conductivity, relatively light weight and relative low fabrication cost. Two forms of aluminum oxide, are however commonly achieved through anodization, the soft or porous aluminum oxide and the hard or barrier aluminum oxide. Both of these oxides are well known in the metallurgical and capacitor arts and have been the subject of extensive characterization. The former of these oxides, the soft oxide has, in fact, been used for metallic protective coatings since the early 1920's while the latter of these oxides the hard or barrier oxide, has been employed as a dielectric material in electrolytic capacitors also since the early 1900's. Materials of the soft oxide turned hard or of the transformed or converted or densified soft oxide type are related to this barrier form of oxide and are herein shown to be capable of enhancing the state of the capacitor dielectric art.